I'm a fan of Badger [1] now as they have the lowest price I've seen that also includes whois privacy ($10).

If you want to use my referral link [2] you can register or transfer a domain for $5.

I'm also a big fan of Hover [3] and Namecheap [4] too. I generally want a no nonsense, no annoying upsells, simple domain registrar. You also should think about what TLD you want as different registrars will have different TLDs especially now that the new GTLDs have arrived.

[1]: http://badger.com [2]: http://badger.com/derek [3]: http://hover.com [4]: http://namecheap.com